Recently, minerals have become recognized as important elements for human health. For example, a deficiency of zinc or manganese causes taste abnormality or growth disorder of the human body. This deficiency is caused by a change in the kinds of food eaten or in eating habits. It is very important to take these minerals every day for preventing diseases.
The oyster, or Crassostrea gigas is well known as a perfect nutritious food, because it contains large amounts of nutritious components compared with other foods. However, it is not advisable to take oysters as daily food, because they tend to cause allergy and because of the accumulation of heavy metals contained in oysters. In addition, the ingredients of the oysters change depending on the season or place from which they were harvested.
To solve these problems, oyster extract tablets have been developed and marketed as a healthy food, which tablets are generally prepared by extracting oysters with water or hot water as disclosed in Japanese Patent Publication Kokoku Hei 4-63672(1992-63672 B2). In this method, an oyster extract is obtained by hot water extraction at 50 to 90° C. for 2 to 3 hours, and ethanol is then added to the extract to obtain a precipitate.
Although water soluble ingredients such as taurine, glycogen, and other substances are extracted with water, water-insoluble minerals such as zinc, manganese, and other minerals can not be extracted by water or hot water. Therefore, large amounts of minerals remain in the residue of the oyster extracted by these means.